Communication cables are typically constructed with an outer protective sheathing consisting of polyethylene or lead. This sheathing serves to protect the underlying metallic shielding and conductor bundle from moisture ingress as well as from physical damage. Many of these cables are pressurized with dried air pumped from a local switching office under low pressure to insure that if leaks do occur in the sheath, a positive pressure within the cable will tend to keep moisture from entering the leaks. However, even with pressurized cable, any significant damage to the cable sheath must be repaired to prevent overburdening the air pressure system. And, in non-pressurized systems or in high moisture areas sheath damage must be repaired to assure that ingress of water is prevented.
Heat shrinkable materials are widely used for the repair of damage to telecommunications cable sheaths. The ability of heat shrinkable materials to achieve a good bond and excellent conformity to cable sheath materials makes them the method of choice where safe practice allows. The heat shrinkable materials typically consist of a polymeric tape or sheet that shrinks when heated and is coated on one surface with a heat activated adhesive for bonding the heat shrinkable material to the cable sheath. Gas torches are currently used to shrink the heat shrinkable material and to activate the adhesive for bonding. The torch, however, produces uneven heating because it only covers a small area at a time and it must be rotated around the cable. This makes the use of the torch dangerous because the operator tends to be pointing the torch at himself during a part of the operation. Moreover, the torch is unusable in cramped spaces or if several cables run together, as in a manhole or cable vault. Also, the torch is usually prohibited from being used in manholes and cable vaults because of the possible danger of igniting built-up explosive gases. Consequently, heat shrinkable materials are not often used in manholes or cable vaults.
A common area at which damage to telephone cable sheath occurs is at the entrance to a manhole or a cable vault from a cable duct. Cables entering the manhole or vault from the cable duct are frequently bent at the entrance area up to 90.degree. causing stress in the cable sheath. This region will often develop cracks and splits in the cable sheath due to the stress. And, even where a torch is permitted in a manhole or cable vault, repairs at the entrance face and inside the duct are inaccessible to a torch.
The prior art includes strip heaters as disclosed in U.S. Pat. Nos. 2,719,907; 3,049,465 and 4,363,947 and German Offenlegungsschrift No. 1,804,649. It also includes other electrical resistance heaters for cylindrical objects such as the heaters disclosed in U.S. Pat. Nos. 2,617,916; 4,032,380; and 4,362,684. However, none of these patents discloses a method of cable sheath repair, none of the heaters they disclose is designed for heating a heat shrinkable material to repair a communications cable, and none of them would be useful in providing cable sheath repair at the cable duct entrance to a manhole or cable vault.